X-ray or gamma ray systems or devices – Specific application – Lithography
Reexamination Certificate
1999-10-07
2001-05-15
Church, Craig E. (Department: 2882)
X-ray or gamma ray systems or devices
Specific application
Lithography
C378S145000
Reexamination Certificate
active
06233306
ABSTRACT:
The invention relates to an apparatus for irradiating an object by means of X-rays, including an X-ray source for producing X-rays for irradiating the object, which X-ray source is provided with a bundle of capillary tubes which conduct X-rays, the end of the bundle which is intended as an exit for the X-rays being provided with an X-ray transparent X-ray window.
An apparatus of this kind is known from European patent No. 0 244 504 B 1.
X-ray irradiation apparatus can be used in a large number of fields of application. A first example of such an application is X-ray analysis where the composition and/or the structure of materials is analyzed. The object to be irradiated is then formed by a specimen of the material to be analyzed by means of the apparatus. Generally speaking, two analysis techniques are feasible: X-ray fluorescence and X-ray diffraction. In the case of X-ray fluorescence, a specimen is irradiated by means of a polychromatic X-ray beam. The irradiation excites the various elements present in the specimen which then emit X-rays (fluorescent radiation) which is characteristic of the constituent elements. The elementary composition of the specimen can be determined by detection and analysis of this fluorescent radiation. In the case of X-ray diffraction, the specimen is generally irradiated by means of a monochromatic X-ray beam which is deflected (diffracted) only at given angles because of the regularity of the crystal structure of the components present in the specimen. The diffraction angles then offer information as regards the crystal structure of the constituents of the specimen.
Another example of a field of application of X-ray irradiation apparatus is X-ray lithography where very small structures for microelectronics are formed on a substrate or masks are manufactured for the exposure of such structures. The object to be irradiated is then formed by said substrate or the mask to be manufactured. Another example of a field of application for X-ray irradiation apparatus is medical therapy or diagnostics where it is often important to apply X-rays to a very accurately defined region of the human body. The object to be irradiated is then formed by the tissue to be irradiated.
In all of said applications the X-rays required for irradiating the object to be examined or treated can be generated by means of an X-ray tube. In such an X-ray tube the X-rays are generated by electron bombardment of an anode so that X-rays are produced in the anode. Because this process must take place in vacuum, the X-ray tube is necessarily constructed so as to include a vacuum tight housing. In order to conduct the X-rays out of the X-ray tube, the housing is provided with a window opening which is situated near the anode and serves to conduct the X-rays produced out of the tube. In generally known conventional X-ray tubes this window opening is covered by an X-ray transparent X-ray window which is usually made of beryllium.
Even though the choice of beryllium as the window material is based on the attractive properties of beryllium in respect of the absorption of X-rays, such absorption cannot be ignored. This holds notably in the case of X-rays having a comparatively long wavelength, for example of the order of magnitude of from 1 nm to 10 nm. It could be attempted to reduce the absorption by the window by making the window thinner, but the strength of the material imposes a limit in this respect. The thickness that can nowadays be achieved for beryllium X-ray windows is of the order of magnitude of 50 &mgr;m. In order to withstand the pressure of the ambient atmosphere on the X-ray window, such thin windows are supported by a supporting grid. Because of the lack of solidity and the high brittleness of beryllium, it is not very likely that these windows can be constructed to be much thinner yet. Other materials for X-ray windows, for example foils of a synthetic material, cannot be used because of the comparatively high temperature whereto the window is exposed during operation of the X-ray tube.
In the apparatus described in the cited European patent No. 0 244 504 capillary tubes for total reflection of X-rays on the interior thereof are combined so as to form a bundle having a length of approximately from 0.5 mm to 1.0 mm. The capillary tubes in this bundle have a diameter of from approximately 10 &mgr;m to 20 &mgr;m, the bundle comprising as many as one hundred thousand capillary tubes so that it has a plate-like external appearance. This plate-like bundle is provided on one side with a thin layer of, for example aluminium or magnesium of a thickness of the order of magnitude of 5 &mgr;m. This thin layer is bombarded by a thin electron beam so that it serves as an X-ray target, the energy of the electron beam being of the order of magnitude of 20 keV. The diameter of the electron beam is approximately 5 &mgr;m, so that it is smaller than the diameter of each of the capillary tubes in the bundle. The other side of the plate-like bundle is provided with a thin layer of, for example beryllium, carbon or a higher polymer with an aluminum coating having a thickness of the order of magnitude of 2 &mgr;m in order to transmit the X-rays generated in the former layer and to intercept any electrons. The latter layer bears on the grid which is formed by the ends of the capillary tubes in the bundle.
Granted, this known structure is suitable for generating X-rays having a comparatively long wavelength. However, generating X-rays of comparatively long wavelength is a process with a low efficiency, i.e. a comparatively high power of the generating electron beam is required so as to generate a low X-ray intensity. Because the thin layer acting as the X-ray target is not provided with cooling means for discharging the heat dissipated in this layer, only a small electric power can be applied to this layer by the electron beam. The X-ray power of this structure, therefore, is very limited.
It is an object of the invention to provide an X-ray irradiation apparatus in which the X-ray source is suitable to produce X-rays of comparatively long wavelength and an intensity which suffices to operate the X-ray irradiation apparatus in practical circumstances. To this end, the apparatus according to the invention is characterized in that the X-ray source includes an X-ray tube having a vacuum tight housing which is provided with a window opening for conducting the X-rays produced by the tube to the exterior of the housing, that one end of the bundle is provided on the window opening in a vacuum tight manner and that the capillary tubes at that end of the bundle are directed towards the location where the X-rays are generated, that the interior of the capillary tubes is in vacuum contact with the vacuum space of the X-ray tube which is situated within the housing, and that the X-ray transparent X-ray window seals the interior of the capillary tubes from the environment in a vacuum tight manner.
A bundle of X-ray conducting capillary tubes is known per se, for example from a contribution to the Proceedings of SPIE, Vol. 3115 (1997), entitled “Polycapillary Focusing Optic For Low-Energy-X-Ray Fluorescence” by Ira Klotzky and Qi-Fan Xiao. The conductive properties of such capillary tubes is based on the well-known phenomenon concerning total reflection of X-rays on the interior of the capillary tubes. Because of the total reflection, only an insignificant loss of intensity occurs, so that these capillary tubes can be used to conduct the X-rays in a loss-free manner. The capillary tubes are assembled so as to form a bundle in known manner in that at one end (the end to be connected to the X-ray tube) of this bundle the capillary tubes are enclosed by a bonding material, for example a synthetic material. Thus, the gaps between the capillary tubes are filled in an airtight manner and at the same time an envelope is formed on the outer side of the bundle; this envelope can also be used for connecting the bundle to the X-ray tube. Such connection can be realized, for example by providing the window opening of t
Church Craig E.
U.S. Philips Corporation
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